1. Field of the Invention
The present invention relates generally to an apparatus and method for measuring stresses in a rock mass, and more specifically, to a method and apparatus employing a strain gauge in a borehole.
2. Description of the Related Art
Stress measurement in rock is carried out extensively in fields such as mining, civil engineering (highways, bridges, dams), and geology. Accurate and appropriate stress data are extremely important because they often play a significant role in the stability of the rock mass or structure being monitored.
Rock stress measurement is commonly performed by drilling a borehole in the rock and installing one or more sensors composed of photoelastic plugs, flat jacks, vibrating wire gauges, or load cells. The signals from these sensors are read by equipment designed to process the data into values indicative of the rock stress, by application of calibration factors or analytically developed equations.
Commercially available borehole stress-meters are deficient in that they are expensive, complicated, and difficult to install and use. For example, conventional meters require highly skilled technicians and/or engineers for proper placement of the sensors and the installation process is very time consuming. Also, conventional stress meters often need to be "bonded" in place for proper operation and are seldom removable for reuse.
A borehole stress-meter and method and apparatus for the installation thereof is described in U.S. Pat. No. 4,542,655. The apparatus and method requires a resiliently radially expandable hollow body within which can be positioned a gauge plug having strain gauges fixed thereto. The interior section of the hollow body is tapered, as is the gauge plug so that advancement of the gauge plug within the hollow body will cause the gauge plug to become prestressed. The hollow body can be inserted into a cylindrical bore drilled in earth material, whose stress is to be measured, by an installation or setting tool. Subsequentially, the gauge plug is advanced within the hollow body so as to prestress the gauge plug. The setting tool is then removed and stress may be measured by an appropriate strain indicator.